1. Field of the Invention
The present invention relates to a large-aperture lens for low-illuminance imaging and, in particular, to a large-aperture lens mounted in a small-sized camera for CCTV or the like used for low-illuminance imaging when cultured microorganisms and the like are being observed.
2. Description of the Prior Art
For example, in the case where cultured microorganisms and the like are being observed, imaging at a low illuminance is required. Various kinds of large-aperture lenses have been known as a lens mounted in a small-sized camera for CCTV or the like used for imaging in this case.
Namely, such a large-aperture lens has an angle of view of about 40 degrees, an F number of about 0.85, and a function for correcting chromatic aberration of light having a wavelength of 450 to 550 nm so that fluorescence can be favorably observed.
Meanwhile, such a large-aperture lens has recently been required to have a compact size and a long back focus.
In the conventional large-aperture lens, the whole length of the lens is long, i.e., about four to five times the focal length thereof, thus lacking compactness. On the other hand, its back focus is short, i.e., about 0.2 to 0.3 times the focal length thereof. Accordingly, in order to satisfy the requirements for securing compactness and back focus, it is necessary to use an aspherical lens, thereby increasing the cost of manufacture.